1. Field of the Invention
The present invention relates to a balanced-type surface acoustic wave filter having a balanced-to-unbalanced conversion function, and more particularly, to a balanced-type surface acoustic wave filter in which first and second longitudinally coupled resonator-type surface acoustic wave filter portions are cascade connected.
2. Description of the Related Art
In recent years, with the reduction in size and weight of communication devices such as portable telephones, surface acoustic wave filters are widely used as bandpass filters which can have a small size. Furthermore, with the reduction in size of communication devices, the combination of constituent components is highly desired. Among such surface acoustic wave filters, a surface acoustic wave filter having a balanced-to-unbalanced conversion function is commonly used.
For example, in Japanese Unexamined Patent Application Publication No. 2002-84164 (Patent Document 1), a longitudinally coupled resonator-type surface acoustic wave filter having a balanced-to-unbalanced conversion function shown in FIG. 9 is disclosed.
As shown in FIG. 9, in a surface acoustic wave filter 300 described in Patent Document 1, an electrode structure shown in FIG. 9 is provided on a surface acoustic wave substrate. That is, a first longitudinally coupled resonator-type surface acoustic wave filter portion 301 and a second longitudinally coupled resonator-type surface acoustic wave filter portion 302 are provided. In the surface acoustic wave filter portion 301, first to third IDTs 303 to 305 are arranged in the surface wave propagation direction. Reflectors 306 and 307 are disposed on both sides in the surface wave propagation direction of an area in which the IDTs 303 to 305 are disposed.
In a similar manner, in the second surface acoustic wave filter portion 302, fourth to sixth IDTs 308 to 310 are disposed along the surface wave propagation direction. Reflectors 311 and 312 are disposed on both sides in the surface wave propagation direction of the IDTs 308 to 310.
One terminal of the first surface acoustic wave filter portion 301 is connected to an unbalanced input terminal 313. The first and second surface acoustic wave filter portions 301 and 302 are cascade connected. Then, one terminal of the IDT 309 of the second surface acoustic wave filter portion 302 is connected to a first balanced output terminal 314 and the other terminal is connected to a second balanced output terminal 315.
Here, the first and second surface acoustic wave filter portions 301 and 302 are cascade connected. Furthermore, a signal flowing in a signal line 316 for connecting the IDT 303 and the IDT 308 is opposite in phase to a signal flowing in a signal line 317 for connecting between the IDTs 305 and 310.
In Japanese Unexamined Patent Application Publication No. 2004-7713 (Patent Document 2), a surface acoustic wave filter with a balanced-to-unbalanced conversion function having an electrode structure shown in FIG. 10 is disclosed. As shown in FIG. 10, a longitudinally coupled resonator-type surface acoustic wave filter 400 is arranged in a similar manner as the surface acoustic wave filter 300 shown in FIG. 9 except that weighting is performed in a second surface acoustic wave filter portion 402 and narrow-pitched electrode-finger portions are provided in the first and second surface acoustic wave filter portions 401 and 402. Accordingly, corresponding reference numerals are used for corresponding portions and their description is omitted.
Here, in IDTs 403 to 405 and IDTs 408 to 410, in a pair of adjacent IDTs, a narrow-pitched electrode-finger portion N is arranged such that the pitch of a plurality of electrode fingers including the outermost electrode finger on the side of the opposite IDT is narrower than the pitch of the main electrode fingers of the respective IDT. The discontinuity in the adjacent portions of the IDTs is compensated for by providing the narrow-pitched electrode-finger portion N and filter characteristics are improved. In addition, in the longitudinally coupled resonator-type surface acoustic wave filter 400, in the two-stage cascade connection-type structure, in the surface acoustic wave filter portion 402 connected to first and second balanced output terminals 414 and 415, in addition to the above-described narrow-pitched electrode-finger portion, weighting by differing the electrode-finger cross-widths is provided.
The balancing is improved such that a signal flowing in one signal line 416 for connecting the first surface acoustic wave filter portion 401 and the second surface acoustic wave filter portion 402 is different in phase from a signal flowing in the other signal line 417.
Furthermore, when the above-described cross-width weighting is provided, the balancing is further improved.
When the above-described surface acoustic wave device having a balanced-to-unbalanced conversion function is a three-port device, wherein, for example, the unbalanced input terminal is port 1 and the balanced output terminals are port 2 and port 3, respectively, the amplitude balancing and phase balancing are defined as follows:Amplitude balancing=|A|  formula (1)A=|20 log (S21)|−|20 log (S31)|Phase balancing=|B|  (2)B=|∠S21−∠S31|Moreover, S21 represents a transfer coefficient from port 1 to port 2 and S31 represents a transfer coefficient from port 1 to port 3. Ideal balancing in the filter characteristics of a surface acoustic wave device is when the amplitude balancing is 0 dB and the phase balancing is 180 degrees in the passband.
As described above, Patent Documents 1 and 2 disclose balanced-type surface acoustic wave filters 300 and 400 of a two-element cascade-connection type in which balancing is improved. However, in these surface acoustic wave filters 300 and 400, there is a problem in that a steep spike-like ripple is generated in the amplitude balancing characteristics and phase balancing characteristics in the passband. Accordingly, the improvement of balancing is not sufficient.